Instructor: Umar KalimNUST Institute of Information Technology Operating Systems File System Interface

Agenda File Concept Access Methods Directory Structure File-System Mounting Summary & Conclusion

File Concept Requirement of persistent storage Contiguous logical address space Types: –Data numeric character binary –Program

File Structure None - sequence of words, bytes Simple record structure –Lines –Fixed length –Variable length Complex Structures –Formatted document –Relocatable load file Who decides: –Operating system –Program

File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system Type – needed for systems that support different types Location – pointer to file location on device Size – current file size Protection – controls who can do reading, writing, executing Time, date, and user identification – data for protection, security, and usage monitoring Information about files are kept in the directory structure, which is maintained on the disk

File Operations File is an abstract data type Create Write Read Reposition within file Delete Truncate Open(F i ) – search the directory structure on disk for entry F i, and move the content of entry to memory Close (F i ) – move the content of entry F i in memory to directory structure on disk Some other examples may be?

Open Files open-file-table Several pieces of data are needed to manage open files: –File pointer: pointer to last read/write location, per process that has the file open –File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it –Disk location of the file: cache of data access information –Access rights: per-process access mode information

Open File Locking Provided by some operating systems and file systems Mediates access to a file Mandatory or advisory: –Mandatory – access is denied depending on locks held and requested –Advisory – processes can find status of locks and decide what to do

File Types – Name, Extension

Access Methods Sequential Access read next write next reset rewrite

Access Methods Direct Access read n write n position to n read next write next rewrite n n = relative block number

Directory Structure A collection of nodes containing information about all files F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk

A Typical File-system Organization partitions and volumes

Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users –Two users can have same name for different files –The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

Tree-Structured Directories

Tree-Structured Directories (Cont) Efficient searching Grouping Capability Current directory (working directory) –cd /spell/mail/prog –type list

Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm Creating a new subdirectory is done in current directory mkdir Example: if in current directory /mail mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

Acyclic-Graph Directories Have shared subdirectories and files

Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list  dangling pointer Solutions: –Backpointers, so we can delete all pointers Variable size records a problem –Backpointers using a daisy chain organization –Entry-hold-count solution New directory entry type –Link – another name (pointer) to an existing file –Resolve the link – follow pointer to locate the file

General Graph Directory There is something wrong with this structure. What is it? and why is it there?

General Graph Directory (Cont.) How do we guarantee no cycles? –Allow only links to file not subdirectories –Garbage collection –Every time a new link is added use a cycle detection algorithm to determine whether it is OK

File System Mounting A file system must be mounted before it can be accessed A unmounted file system (i.e. Fig (b)) is mounted at a mount point

(a) Existing. (b) Unmounted Partition

Mount Point

Instructor: Umar KalimNUST Institute of Information Technology Questions? Recommended Reading: –OSRC –Reading list &